Conventionally, there has been known an image reading apparatus, in which a so-called flat bed scanner for scanning the image of a document by mounting an image sensor on a carriage built in a document placing table. The image reading apparatus is mounted on a copier or a multi function device. Also, a CIS (Contact Image Sensor) is known as the image sensor. In this CIS, the photoelectric conversion elements are arrayed in a main scanning direction thereby to output image signals according to the intensities of light beams received. Moreover, the photoelectric conversion elements arranged in the main scanning direction are controlled to output the image signals sequentially in one direction.
From the image sensor in the image reading apparatus, the image signals are sequentially outputted in the main scanning direction from the photoelectric conversion elements so that the scanning of the image of one line takes a time for outputting the image signals sequentially from the photoelectric conversion elements of one line. Proposals have been made to speed up the image reading apparatus by shortening the time period required for reading the images of one line with that image sensor. For example, JP-A-6-189080 discloses a configuration, in which the image signals are outputted in parallel by dividing the photoelectric conversion elements of the image sensor into the element groups.